On Saturday, March 14, 2015 at 5:07:37 AM UTC-4, Jeff wrote:
On 14/03/2015 01:48, Channel Jumper wrote:
Tom W3TDH;836715 Wrote:
I am looking for recommendations for a dual band antenna that will serve
two separate radios. The reason that I want to use a single antenna is
that I have a limited number of mounting points for antennas. At
present it will be hard for SWMBO; as in Rumpole's spouse, She Who Must
Be Obeyed; to tolerate the use of both our home's gable ends and the
chimney being used to support antennas. I am planning to replace my
Diamond X-30 with a triband vertical for Six, Two, and .7 Meters. The
chimney will then support a rotor aimed Two and .7 Meter beam. The
second gable end will support the Two meter / Seventy Centimeter dual
band vertical that I am asking for help in selecting.

One radio will be a two meter packet node which will be used as a
Winlink Radio Message Server. The other radio will be a UHF D-STAR
hotspot. I am willing to pay what is needed to to get the best antenna
for this application but I don't want to waste money ineffectively. So
the two meter radio will be in the 144 MHz portion of the band and I
don't yet actually know were the D-STAR hotspot will be run. I have a
DCI filter and diplexer to keep the two radios from actually knowing of
each others existence. Since a hotspot is not supposed to be a terribly
wide area installation I would imagine that I do not want an extremely
high gain antenna but I am perfectly open to be reeducated on that. The
difficulty is that I would guess that the Radio Message Server / Packet
would benefit from as much horizontal gain as can be achieved. I have a
home brewed collinear two meter J-Pole that has been a good performer on
two meters and presents a low SWR on 440 MHz. I have yet to master
antenna modeling but I would imagine; given all the warnings I have read
on line; that it has poor radiation pattern on UHF. Is it likely to be
too poor a performer for a hot spot on UHF?

I really am asking because I want to know. I am not looking for
encouragement to do something that will be ineffective. Thank you in
advance for any help you may be willing to offer.

--
Tom Horne W3TDH

Tom,
You have too many of the same band antenna's concentrated in one place.
No matter how hard you try, they aren't going to play nice with each
other.
I read 14 posts and no one gave an even close right answer to your
question.
The answer is that you need to construct a tower, no less than 100',
near your house in order to even try to do what you wish to do.
Even then, you will need feet of separation vertically in order to get
the antenna's to play nice with each other.
Your SWR is going to be all messed up, because you have too many of the
same antenna's in the same proximity.

The only good 6 / 2 / 70 cm antenna that I could recommend would be the
Diamond v2000. This is the only antenna that I am aware of that has a
decent amount of gain - if you want to call it that, along with being
semi resonant on all three bands.

What you are doing is back-feeding everything that you transmit back
into the receive of the front ends of all of the radios in your shack
when any one radio transmits. Unless it is in your budget to replace
all those radios on a semi annual schedule, you will eventually
experience that each of those transceivers will eventually become deaf.

I have seen filters promoted in QST that allows two operators on two
different bands to share a beam antenna with two transceivers, as long
as each transceiver stays on it's band it is ok.

But there is a hell of a difference between 20 meters - 14 MHz - CW and
40 meters Phone.

Even though 70 cm is not a harmonic of 2m, and even though there is a
heck of a disparity between 440 MHz and 146 MHZ there is always going to
be problems when dealing with FM, and Digital modes.

I have to take your wife's side on this one!

Tell your club to go out and buy an acre of ground and put up a
transmitter and a tower and put their packet and their D-Star crap on
their tower, and then you can tune to their tower frequency if you so
choose. You are killing not only all of your transceivers by what you
are trying to do, but you are diminishing the range at which you
yourself can operate...

If you can hear other repeaters / more than 20 miles away though all of
that RF noise you have created, you will be lucky..

It doesn't matter if the radios are all turned on or off, as long as
they are connected to the coax / antenna, they are still going to
experience front end overload.


What a complete load of hog wash!!

A dual band antenna and the DCI filter/diplexer will work just fine.
The DCI filter provides bandpass characteristics on both 2 & 70 and then
provides a single output. The filters provide more than adequate
isolation between radios to stop any damage for reasonable power levels.

You almost certainly will be able to hear the harmonics from the 2m tx
on 70 but the level will not cause any damage.

By the way Channel Jumper 70cms IS a harmonic of 2m, perhaps time to
brush up on your 3 times table.

Jeff



On Saturday, March 14, 2015 at 5:07:37 AM UTC-4, Jeff wrote:
On 14/03/2015 01:48, Channel Jumper wrote:
Tom W3TDH;836715 Wrote:
I am looking for recommendations for a dual band antenna that will serve
two separate radios. The reason that I want to use a single antenna is
that I have a limited number of mounting points for antennas. At
present it will be hard for SWMBO; as in Rumpole's spouse, She Who Must
Be Obeyed; to tolerate the use of both our home's gable ends and the
chimney being used to support antennas. I am planning to replace my
Diamond X-30 with a triband vertical for Six, Two, and .7 Meters. The
chimney will then support a rotor aimed Two and .7 Meter beam. The
second gable end will support the Two meter / Seventy Centimeter dual
band vertical that I am asking for help in selecting.

One radio will be a two meter packet node which will be used as a
Winlink Radio Message Server. The other radio will be a UHF D-STAR
hotspot. I am willing to pay what is needed to to get the best antenna
for this application but I don't want to waste money ineffectively. So
the two meter radio will be in the 144 MHz portion of the band and I
don't yet actually know were the D-STAR hotspot will be run. I have a
DCI filter and diplexer to keep the two radios from actually knowing of
each others existence. Since a hotspot is not supposed to be a terribly
wide area installation I would imagine that I do not want an extremely
high gain antenna but I am perfectly open to be reeducated on that. The
difficulty is that I would guess that the Radio Message Server / Packet
would benefit from as much horizontal gain as can be achieved. I have a
home brewed collinear two meter J-Pole that has been a good performer on
two meters and presents a low SWR on 440 MHz. I have yet to master
antenna modeling but I would imagine; given all the warnings I have read
on line; that it has poor radiation pattern on UHF. Is it likely to be
too poor a performer for a hot spot on UHF?

I really am asking because I want to know. I am not looking for
encouragement to do something that will be ineffective. Thank you in
advance for any help you may be willing to offer.

--
Tom Horne W3TDH

Tom,
You have too many of the same band antenna's concentrated in one place.
No matter how hard you try, they aren't going to play nice with each
other.
I read 14 posts and no one gave an even close right answer to your
question.
The answer is that you need to construct a tower, no less than 100',
near your house in order to even try to do what you wish to do.
Even then, you will need feet of separation vertically in order to get
the antenna's to play nice with each other.
Your SWR is going to be all messed up, because you have too many of the
same antenna's in the same proximity.

The only good 6 / 2 / 70 cm antenna that I could recommend would be the
Diamond v2000. This is the only antenna that I am aware of that has a
decent amount of gain - if you want to call it that, along with being
semi resonant on all three bands.

What you are doing is back-feeding everything that you transmit back
into the receive of the front ends of all of the radios in your shack
when any one radio transmits. Unless it is in your budget to replace
all those radios on a semi annual schedule, you will eventually
experience that each of those transceivers will eventually become deaf.

I have seen filters promoted in QST that allows two operators on two
different bands to share a beam antenna with two transceivers, as long
as each transceiver stays on it's band it is ok.

But there is a hell of a difference between 20 meters - 14 MHz - CW and
40 meters Phone.

Even though 70 cm is not a harmonic of 2m, and even though there is a
heck of a disparity between 440 MHz and 146 MHZ there is always going to
be problems when dealing with FM, and Digital modes.

I have to take your wife's side on this one!

Tell your club to go out and buy an acre of ground and put up a
transmitter and a tower and put their packet and their D-Star crap on
their tower, and then you can tune to their tower frequency if you so
choose. You are killing not only all of your transceivers by what you
are trying to do, but you are diminishing the range at which you
yourself can operate...

If you can hear other repeaters / more than 20 miles away though all of
that RF noise you have created, you will be lucky..

It doesn't matter if the radios are all turned on or off, as long as
they are connected to the coax / antenna, they are still going to
experience front end overload.


What a complete load of hog wash!!

A dual band antenna and the DCI filter/diplexer will work just fine.
The DCI filter provides bandpass characteristics on both 2 & 70 and then
provides a single output. The filters provide more than adequate
isolation between radios to stop any damage for reasonable power levels.

You almost certainly will be able to hear the harmonics from the 2m tx
on 70 but the level will not cause any damage.

By the way Channel Jumper 70cms IS a harmonic of 2m, perhaps time to
brush up on your 3 times table.

Jeff

Jeff

May I ask that you cool your jets a little. Please turn off the afterburners and stick to the questions that I'm asking. I don't want to be a focus of a flame war. I came here in the hope of getting good information and the task of sorting out conflicting advice is so much easier if I don't need to also filter out gratuitous feuding.

Your version of what is possible is attractive because it allows me to do what I want to do to support my clubs D-STAR project and provide a training resource for the ARES Hospital Emergency Net staff to become competent with Winlink.

I realize that there are some Hams that despise Winlink and all other automated store and forward systems. I will be doing everything I can to avoid interference to other Amateurs but since we are using this to support hospitals under emergency conditions I think that everyone can just suck it up and bare with the emergency traffic and whatever inconvenience it may generate. b

Would you please advise if the third harmonic issue is likely to be serious enough to require the use of a low pass filter between the two meter transceiver and the Diplexer/Band Pass Filter when using separate transmitters?

--
Tom Horne W3TDH

Tom,

Please see inlined...

On 3/14/2015 1:17 PM, Tom W3TDH wrote:

May I ask that you cool your jets a little. Please turn off the
afterburners and stick to the questions that I'm asking. I don't
want to be a focus of a flame war. I came here in the hope of
getting good information and the task of sorting out conflicting
advice is so much easier if I don't need to also filter out
gratuitous feuding.

If you followed this group before posting (always a good idea), you will
see that is pretty normal here. If you're going to post, you can expect
both good and bad advice. And some people (like Channel Jumper - who
probably isn't even a ham) are so far off with every post you can expect
a flame war.

But then that is true of almost every place on the Internet.

Your version of what is possible is attractive because it allows me
to do what I want to do to support my clubs D-STAR project and
provide a training resource for the ARES Hospital Emergency Net staff
to become competent with Winlink.

I realize that there are some Hams that despise Winlink and all other
automated store and forward systems. I will be doing everything I can
to avoid interference to other Amateurs but since we are using this to
support hospitals under emergency conditions I think that everyone can
just suck it up and bare with the emergency traffic and whatever
inconvenience it may generate. b


You are required by law to avoid interference. But you also need to
understand what constitutes emergency traffic to the FCC. For that to
occur, there must be an IMMEDIATE threat to life and/or property. An
example would be reporting an automobile accident, especially one
requiring immediate medical attention.

It does NOT include 99% of the traffic being passed during an emergency.
In the 19 years I've been a member of MoCo ARES/RACES (including the
Hospital Net), we have NEVER had emergency traffic passed during a
callout. Of course, I could also say that of most of the emergencies
I've been involved in in 47 years as a ham. The only times I can think
of that I've been involved in true emergency traffic during a callout
was when aiding search and rescue after tornadoes.

Just because it's a hospital net does not constitute emergency traffic.

So, while hams in the area will voluntarily yield the frequency, there
is no requirement for them to do so, and any interference would be a
violation.

Would you please advise if the third harmonic issue is likely to be
serious enough to require the use of a low pass filter between the
two meter transceiver and the Diplexer/Band Pass Filter when using
separate transmitters?


Your diplexer should handle that well enough. What would be more of a
concern to me would be just the spurious radiation around the shack.
It's possible that could cause some desense, but I wouldn't expect it to
be significant.

--
Tom Horne W3TDH


Your sig separator is broken. It needs to be exactly
hyphen-hyphen-space-newline. You're missing the space.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On Saturday, March 14, 2015 at 3:56:38 PM UTC-4, Jerry Stuckle wrote:
Tom,

Please see inlined...

On 3/14/2015 1:17 PM, Tom W3TDH wrote:

May I ask that you cool your jets a little. Please turn off the
afterburners and stick to the questions that I'm asking. I don't
want to be a focus of a flame war. I came here in the hope of
getting good information and the task of sorting out conflicting
advice is so much easier if I don't need to also filter out
gratuitous feuding.

If you followed this group before posting (always a good idea), you will
see that is pretty normal here. If you're going to post, you can expect
both good and bad advice. And some people (like Channel Jumper - who
probably isn't even a ham) are so far off with every post you can expect
a flame war.

But then that is true of almost every place on the Internet.

Your version of what is possible is attractive because it allows me
to do what I want to do to support my clubs D-STAR project and
provide a training resource for the ARES Hospital Emergency Net staff
to become competent with Winlink.

I realize that there are some Hams that despise Winlink and all other
automated store and forward systems. I will be doing everything I can
to avoid interference to other Amateurs but since we are using this to
support hospitals under emergency conditions I think that everyone can
just suck it up and bare with the emergency traffic and whatever
inconvenience it may generate. b


You are required by law to avoid interference. But you also need to
understand what constitutes emergency traffic to the FCC. For that to
occur, there must be an IMMEDIATE threat to life and/or property. An
example would be reporting an automobile accident, especially one
requiring immediate medical attention.

It does NOT include 99% of the traffic being passed during an emergency.
In the 19 years I've been a member of MoCo ARES/RACES (including the
Hospital Net), we have NEVER had emergency traffic passed during a
callout. Of course, I could also say that of most of the emergencies
I've been involved in in 47 years as a ham. The only times I can think
of that I've been involved in true emergency traffic during a callout
was when aiding search and rescue after tornadoes.

Just because it's a hospital net does not constitute emergency traffic.

So, while hams in the area will voluntarily yield the frequency, there
is no requirement for them to do so, and any interference would be a
violation.

Would you please advise if the third harmonic issue is likely to be
serious enough to require the use of a low pass filter between the
two meter transceiver and the Diplexer/Band Pass Filter when using
separate transmitters?


Your diplexer should handle that well enough. What would be more of a
concern to me would be just the spurious radiation around the shack.
It's possible that could cause some desense, but I wouldn't expect it to
be significant.

--
Tom Horne W3TDH


Your sig separator is broken. It needs to be exactly
hyphen-hyphen-space-newline. You're missing the space.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

Jerry

All I was trying to do by saying that we plan to be careful about interference is to calm and reassure those that are pathologically afraid of Winlink.. I do realize that nothing short of a true SOS like being pinned under a fallen snag with a widfire approaching qualifies as an emergency in the eyes of the Friendly Candy Company (FCC). They have been known to confiscate radios and cancel the license of hams who invaded a public safety channel to ask for help with a person who was bleeding profusely from a scalp wound. As a trained EMT I know that scalp wounds are often scarier than they look but how a ham out mountain biking with friends was supposed to know that I have no idea. The actual reaction of the FCC to any given situation is just as predictable as the reaction of any other organization that is staffed by human beings.

Thanks for pointing out the the problem with my signature element. I appreciate the help.

--
Tom Horne W3TDH

On 3/15/2015 11:25 AM, Tom W3TDH wrote:
On Saturday, March 14, 2015 at 3:56:38 PM UTC-4, Jerry Stuckle wrote:
Tom,

Please see inlined...

On 3/14/2015 1:17 PM, Tom W3TDH wrote:

May I ask that you cool your jets a little. Please turn off the
afterburners and stick to the questions that I'm asking. I don't
want to be a focus of a flame war. I came here in the hope of
getting good information and the task of sorting out conflicting
advice is so much easier if I don't need to also filter out
gratuitous feuding.

If you followed this group before posting (always a good idea), you will
see that is pretty normal here. If you're going to post, you can expect
both good and bad advice. And some people (like Channel Jumper - who
probably isn't even a ham) are so far off with every post you can expect
a flame war.

But then that is true of almost every place on the Internet.

Your version of what is possible is attractive because it allows me
to do what I want to do to support my clubs D-STAR project and
provide a training resource for the ARES Hospital Emergency Net staff
to become competent with Winlink.

I realize that there are some Hams that despise Winlink and all other
automated store and forward systems. I will be doing everything I can
to avoid interference to other Amateurs but since we are using this to
support hospitals under emergency conditions I think that everyone can
just suck it up and bare with the emergency traffic and whatever
inconvenience it may generate. b


You are required by law to avoid interference. But you also need to
understand what constitutes emergency traffic to the FCC. For that to
occur, there must be an IMMEDIATE threat to life and/or property. An
example would be reporting an automobile accident, especially one
requiring immediate medical attention.

It does NOT include 99% of the traffic being passed during an emergency.
In the 19 years I've been a member of MoCo ARES/RACES (including the
Hospital Net), we have NEVER had emergency traffic passed during a
callout. Of course, I could also say that of most of the emergencies
I've been involved in in 47 years as a ham. The only times I can think
of that I've been involved in true emergency traffic during a callout
was when aiding search and rescue after tornadoes.

Just because it's a hospital net does not constitute emergency traffic.

So, while hams in the area will voluntarily yield the frequency, there
is no requirement for them to do so, and any interference would be a
violation.

Would you please advise if the third harmonic issue is likely to be
serious enough to require the use of a low pass filter between the
two meter transceiver and the Diplexer/Band Pass Filter when using
separate transmitters?


Your diplexer should handle that well enough. What would be more of a
concern to me would be just the spurious radiation around the shack.
It's possible that could cause some desense, but I wouldn't expect it to
be significant.

--
Tom Horne W3TDH


Your sig separator is broken. It needs to be exactly
hyphen-hyphen-space-newline. You're missing the space.


Jerry

All I was trying to do by saying that we plan to be careful about interference is to calm and reassure those that are pathologically afraid of Winlink. I do realize that nothing short of a true SOS like being pinned under a fallen snag with a widfire approaching qualifies as an emergency in the eyes of the Friendly Candy Company (FCC). They have been known to confiscate radios and cancel the license of hams who invaded a public safety channel to ask for help with a person who was bleeding profusely from a scalp wound. As a trained EMT I know that scalp wounds are often scarier than they look but how a ham out mountain biking with friends was supposed to know that I have no idea. The actual reaction of the FCC to any given situation is just as predictable as the reaction of any other organization that is staffed by human beings.

Thanks for pointing out the the problem with my signature element. I appreciate the help.


Tom,

From your previous statement:

"I will be doing everything I can to avoid interference to other
Amateurs but since we are using this to support hospitals under
emergency conditions I think that everyone can just suck it up and bare
with the emergency traffic and whatever inconvenience it may generate."

I'm just pointing out that by law you are REQUIRED to avoid
interference. Also, your traffic is almost NEVER emergency traffic, and
any interference you cause will be illegal. Other hams do NOT have to
"suck it up" just because you're calling it a "hospital net". If you
cause interference, you are liable - unless it is a true emergency.

And yes, I was part of the local hospital net before you ever got your
license. I manned Suburban Hospital. Of course, we had an 800MHz
channel at the time, not ham. But the hams were the operators of the
system.

BTW - I also was an EMT - and probably have worked more runs than you've
ever dreamed. I know how bloody a scalp wound can be. But I also know
it can be a symptom of a much more serious injury such as a skull
fracture (BTDT). To a non-trained person, this would be emergency
traffic. As an EMT, I would never consider such a wound to NOT be
emergency traffic when reported by a non-trained person. Neither would
any of the other EMS people I have known over the years.

And I'd like to see the details of the "hams who invaded a public safety
channel to ask for help with a person who was bleeding profusely from a
scalp wound". I suspect there is much more to it than you are
reporting. References?

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On 3/15/2015 5:50 AM, Brian Reay wrote:
Tom W3TDH wrote:


Would you please advise if the third harmonic issue is likely to be
serious enough to require the use of a low pass filter between the two
meter transceiver and the Diplexer/Band Pass Filter when using separate transmitters?


It depends on your tx.

I've never required filters nor heard of their being required.

Remember, you would need to be listening on the 3rd Rx harmonic, or 'close'
to it for the rx to suffer and the harmonic should get several 10s of dB
down relative to tell 2m carrier.


Brian,

Yes, it will be several 10s of dB down - but that can still be
sufficient to be heard in (and potentially desense) a near-by receiver.
It doesn't take a lot of signal if it's within the receiver's passband.

But Tom's diplexer should handle this easily.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On 15/03/15 13:33, Jerry Stuckle wrote:
On 3/15/2015 5:50 AM, Brian Reay wrote:
Tom W3TDH wrote:


Would you please advise if the third harmonic issue is likely to be
serious enough to require the use of a low pass filter between the two
meter transceiver and the Diplexer/Band Pass Filter when using separate transmitters?


It depends on your tx.

I've never required filters nor heard of their being required.

Remember, you would need to be listening on the 3rd Rx harmonic, or 'close'
to it for the rx to suffer and the harmonic should get several 10s of dB
down relative to tell 2m carrier.


Brian,

Yes, it will be several 10s of dB down - but that can still be
sufficient to be heard in (and potentially desense) a near-by receiver.
It doesn't take a lot of signal if it's within the receiver's passband.

But Tom's diplexer should handle this easily.


Well, I several of us often run full duplex 2m/70cm or even triplex
4m/2m/70cm nets locally and use a common 2m/70cm antenna without
problems- hence my comment that I've never experienced any issues.
(Being in the UK, we have access to 4m.) I've even run 2m/70cm crossband
duplex while mobile with a common antenna and with dual antennas on the
roof of a SUV. True, I've not used the harmonic of the 2m frequency but
the desense was never an issue. While mobile and using the single
antenna, I relied on the internal duplexer of the radio. When using two
antennas, there was no filter or duplexer added.

If you think about it from a practical stand point, dual band radio with
the ability to operate full duplex cross band would be pretty useless if
it was an issue.

I'm simply relating my experiences of actually using 2m and 70cm on a
common antenna with a duplexer. If your experiences differ, fine.

On 3/15/2015 11:54 AM, Brian Reay wrote:


Well, I several of us often run full duplex 2m/70cm or even triplex
4m/2m/70cm nets locally and use a common 2m/70cm antenna without
problems- hence my comment that I've never experienced any issues.
(Being in the UK, we have access to 4m.) I've even run 2m/70cm crossband
duplex while mobile with a common antenna and with dual antennas on the
roof of a SUV. True, I've not used the harmonic of the 2m frequency but
the desense was never an issue. While mobile and using the single
antenna, I relied on the internal duplexer of the radio. When using two
antennas, there was no filter or duplexer added.

If you think about it from a practical stand point, dual band radio with
the ability to operate full duplex cross band would be pretty useless if
it was an issue.

I'm simply relating my experiences of actually using 2m and 70cm on a
common antenna with a duplexer. If your experiences differ, fine.





Brian,

Have you actually measured desense with good test equipment? If not,
you really don't know, do you?

I've run 2m/70cm crossband mobile also. However, I also haven't
measured the desense when trying to receive on the third harmonic. Have
you?

Additionally, when running crossband mobile, I've always gone from a
nearby handheld on 70cm to a 2m repeater. Never tried it the other way
around, and since the handheld is always (relatively) nearby, it always
has a strong signal into the mobile. So even if there were desense I
wouldn't see it.

--
==================
Remove the "x" from my email address
Jerry, AI0K

==================

On 15/03/15 16:40, Jerry Stuckle wrote:
On 3/15/2015 11:54 AM, Brian Reay wrote:


Well, I several of us often run full duplex 2m/70cm or even triplex
4m/2m/70cm nets locally and use a common 2m/70cm antenna without
problems- hence my comment that I've never experienced any issues.
(Being in the UK, we have access to 4m.) I've even run 2m/70cm crossband
duplex while mobile with a common antenna and with dual antennas on the
roof of a SUV. True, I've not used the harmonic of the 2m frequency but
the desense was never an issue. While mobile and using the single
antenna, I relied on the internal duplexer of the radio. When using two
antennas, there was no filter or duplexer added.

If you think about it from a practical stand point, dual band radio with
the ability to operate full duplex cross band would be pretty useless if
it was an issue.

I'm simply relating my experiences of actually using 2m and 70cm on a
common antenna with a duplexer. If your experiences differ, fine.





Brian,

Have you actually measured desense with good test equipment? If not,
you really don't know, do you?

I've run 2m/70cm crossband mobile also. However, I also haven't
measured the desense when trying to receive on the third harmonic. Have
you?

Additionally, when running crossband mobile, I've always gone from a
nearby handheld on 70cm to a 2m repeater. Never tried it the other way
around, and since the handheld is always (relatively) nearby, it always
has a strong signal into the mobile. So even if there were desense I
wouldn't see it.


I haven't measured the desense, I was relating practical experience as I
said. In the circumstances, they are probably more relevant in my
opinion*. Tom is free to take which ever advice he decides is relevant.

* The desense would have a number of variables, including the tx power
(which may vary), tx and rx frequency (also variable). Plus the impact
of the dense on the receive performance is almost impossible to predict
in terms of who/what you could receive- by which I mean you could
generate a raft of numbers for loss of system sensitivity but how do you
translate that into practical link budgets, other than for known
stations? It would be virtually useless for a random contact.



73
Brian

On 3/15/2015 4:47 PM, Brian Reay wrote:
On 15/03/15 16:40, Jerry Stuckle wrote:
On 3/15/2015 11:54 AM, Brian Reay wrote:


Well, I several of us often run full duplex 2m/70cm or even triplex
4m/2m/70cm nets locally and use a common 2m/70cm antenna without
problems- hence my comment that I've never experienced any issues.
(Being in the UK, we have access to 4m.) I've even run 2m/70cm crossband
duplex while mobile with a common antenna and with dual antennas on the
roof of a SUV. True, I've not used the harmonic of the 2m frequency but
the desense was never an issue. While mobile and using the single
antenna, I relied on the internal duplexer of the radio. When using two
antennas, there was no filter or duplexer added.

If you think about it from a practical stand point, dual band radio with
the ability to operate full duplex cross band would be pretty useless if
it was an issue.

I'm simply relating my experiences of actually using 2m and 70cm on a
common antenna with a duplexer. If your experiences differ, fine.





Brian,

Have you actually measured desense with good test equipment? If not,
you really don't know, do you?

I've run 2m/70cm crossband mobile also. However, I also haven't
measured the desense when trying to receive on the third harmonic. Have
you?

Additionally, when running crossband mobile, I've always gone from a
nearby handheld on 70cm to a 2m repeater. Never tried it the other way
around, and since the handheld is always (relatively) nearby, it always
has a strong signal into the mobile. So even if there were desense I
wouldn't see it.


I haven't measured the desense, I was relating practical experience as I
said. In the circumstances, they are probably more relevant in my
opinion*. Tom is free to take which ever advice he decides is relevant.


So you really don't know if you have any desense or not. You just
haven't noticed any. That does not mean it's not there.

* The desense would have a number of variables, including the tx power
(which may vary), tx and rx frequency (also variable). Plus the impact
of the dense on the receive performance is almost impossible to predict
in terms of who/what you could receive- by which I mean you could
generate a raft of numbers for loss of system sensitivity but how do you
translate that into practical link budgets, other than for known
stations? It would be virtually useless for a random contact.



73
Brian


Actually, knowing the characteristics of the transmitter and receiver,
as well as the characteristics of whatever filters are used, it's
possible to quite accurately determine desense. That allows you to
predict with pretty good accuracy how much signal it would take to
attain a certain signal level (i.e. 12db SINAD) in the receiver. But
that's the important part of ANY system.

And it's impossible to predict the results of ANY random contact without
knowing the details of the other station. But then that's true on any
frequency with any mode.

Something I did a fair amount of back in the mid 70's when I worked for
a worked for a 2-way shop and planned UHF repeater systems. I doubt
it's changed any today.

--
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Jerry, AI0K

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